The present invention relates to a method for the preparation of a shaped article of a vinyl chloride-based resin having improved surface properties. More particularly, the invention relates to a method for the preparation of a shaped article of a vinyl chloride-based resin, of which the undesirable phenomenon of bleeding or exudation of the plasticizer and other additive ingredients contained therein is greatly reduced by imparting improved properties to the surface thereof.
As is well known, polyvinyl chloride resins have found wide applications owing to their large versatility in the rigidity or flexibility of the articles shaped therewith by formulating with or without a plasticizer. Their applications extend to the fields not only where rigid shaped articles are required but also where flexibility of the shaped articles is an essential condition including films, sheets, synthetic leathers, tubes, hoses, bags, packing materials, coating materials and the like to be used as a medical tool, wrapping material for foods, insulating material for electric wires, material for agricultural use, building material and the like.
One of the difficult problems in the plasticized, flexible shaped articles of a vinyl chloride-based resin is the mobility of the plasticizer contained therein that the plasticizer migrates toward and exudes on the surface of the article in the long run. This phenomenon is called bleeding.
Bleeding or exudation of the plasticizer is undesirable because of the less pleasant appearance, denaturation and decreased durability of the shaped article along with the problem of safety due to the transfer of the plasticizer, which may be toxic more or less, from the shaped article to a second body in contact with the shaped article. Therefore, the application of a plasticized shaped article of a vinyl chloride-based resin is largely limited, in particular, for medical and foodstuff use. There has been proposed a method for overcoming the above described problem, according to which the surface of the plasticized shaped article of a polyvinyl chloride resin is irradiated with ultraviolet light predominantly in the wavelength region of 200 nm or shorter so as that a high-density crosslinked layer is formed on or in the vicinity of the surface of the shaped article to be effective to prevent migration and exudation of the plasticizer and other additive ingredients on the surface of the shaped article (see, for example, Japanese Patent Disclosure 54-64573).
The above described method of the treatment with the so-called vacuum-ultraviolet light still has a problem that the effect of the treatment is not always sufficiently lasting but the crosslinked layer once formed on the surface of the article is subject to degradation when the article is subsequently irradiated with ultraviolet light of longer, say, 290 nm or longer, wavelengths or when the article is subjected to outdoor exposure for a long period of time to return to the surface condition before the vacuum-ultraviolet irradiation with consequent bleeding of the plasticizer and the like in the same degree as before the treatment.